1. Field of the Invention
The present invention relates to a mechanical and electrical connection system between the ends of two approximately coaxial shafts, which move along an overall axial direction and that can transmit approximately axial forces. This type of configuration is used particularly when it is required to separate a shaft in two parts, to protect one of its ends from significant conditions (temperatures, stresses, lateral shocks, etc.) endured by the other end.
This invention more particularly relates to crustbreaking devices used in electrolytic aluminium production pots. These are used to break the surface crust of the solidified bath. They are often associated either with metering devices that supply alumina to the pot, or to measurement devices used to measure the temperature and level of electrolyte in the pot so as to enable regulated supply of the bath with alumina.
2. Description of the Related Art
ALUMINIUM PECHINEY patent FR 2 483 965 describes such a crustbreaking device in which the crustbreaker, also called a plunger, is associated with an axial displacement mechanism. It is placed vertically, and periodically descends to break the solidified bath crust at the supply point or to keep the hole formed in the crust open, such that the alumina brought in through a pipe can penetrate into the molten electrolyte. For several reasons explained in FR 2 483 965, it is important to detect if the plunger comes into contact with molten electrolyte or with solidified electrolyte during its downward movement. Detection of the contact between the end of the plunger (called the chisel) and the molten electrolyte is made by measuring an electrical voltage between the plunger and a point on the cell used as the reference potential. Typically, the plunger is electrically isolated from the superstructure of the pot, and when the chisel comes into contact with the molten electrolyte bath, its electrical potential is close to the potential of the molten electrolyte. The measurement of this potential is used as information to send the order to raise the plunger.
Patent EP-B-0 716 165 describes a crustbreaking device surrounding and protecting a measurement device that not only measures the bath level but also its temperature. For this device, it is also important to detect the moment at which the plunger comes into contact with the molten electrolyte, and the crustbreaker is also provided with a detection device based on measurement of an electrical voltage between the plunger and a point on the cell used as a reference potential.
The plunger is subjected to violent thermal and mechanical conditions near the chisel. As the time that the chisel remains in the electrolyte bath increases, the risk of formation of a crust on its surface as it rises increases, and this crust can get thicker during subsequent operations, causing problems with operation and premature wear of the plunger.
The plunger must not be excessively deformed and must be electrically reliable, otherwise it will not operate correctly; it must remain electrically isolated from the pot superstructure at all times and it must always be able to carry an electrical current to detect contact with the electrolyte bath. Furthermore, since the chisel can wear quickly and deform progressively, it is important to provide a means of replacing it easily and regularly. Finally, the other end of the plunger is connected to an axial movement control device, typically a jack, that can only operate correctly at a temperature close to ambient temperature and provided that no excessive mechanical loads are applied to it, since it is particularly sensitive to lateral shocks.
For all these reasons, the plunger has been decoupled into two approximately coaxial parts; a rod associated with the control jack and a rod (called the extension) that is fitted with the chisel at one of its ends. These two parts are usually connected to each other by a screw-nut system but this system is not fully satisfactory since it transmits mechanical shocks resisted by the chisel without really damping them, particularly sudden lateral forces that prevent smooth operation of the jack. Furthermore, loosening problems frequently occur with the screw-nut system (for example when the chisel has to be replaced) and cannot guarantee a reliable permanent electrical contact.
Therefore, the applicant has developed a connecting system between the two parts of the plunger that does not have these disadvantages.